The present invention pertains to the art of machines and, more particularly, to a method and apparatus for determining clearance between a moving member and a static member of a machine.
In the art of rotating machines, it is often necessary to measure clearance between moving or rotating and static components. Without proper clearance, the rotating component may contact the static component. Contacts and, in particular, high speed contacts can result in damage to one or the other of the rotating component and static component. In certain cases, measuring clearance is a minimally invasive procedure. However, in most cases, measuring clearance requires a certain level of machine disassembly or tear down. When the machine is in operation, clearances change due to heat, centrifugal forces and the like. Thus, static clearance must be set with a certain level of flexibility to allow for any changes that may occur. Heat, fatigue and other conditions serve to change operational clearance between moving and static components. In order to ensure that the machine is operating within tolerance, periodic maintenance is required to measure and adjust the clearance. Given the need, in most cases, for at least partial disassembly to measure clearance, maintenance requires that the machine be taken offline. Taking a machine offline is a time consuming and costly procedure that will have a negative impact on facility's production output.
In some cases, microwaves are employed to measure a running or moving clearance between turbine components. In such a case, a microwave transmitter is mounted to a moving component and a receiver is mounted to a static component in a turbine. A microwave is then passed between the receiver and transmitter to determine clearance. While effective in most cases, microwaves are affected by water. Given the interaction between microwaves and water, a microwave system for detection clearance would not be particularly effective in turbines in which the working fluid has a high moisture content, such as, for example, a steam turbine.